This invention relates to a coordinate input apparatus for generating coordinates corresponding to a beam spot produced by a designating tool.
Known examples of an input apparatus according to the prior art include one in which a beam spot on a screen is sensed using a CCD area sensor or linear CCD sensor and image processing using coordinates of the center of gravity or pattern matching is executed to calculate and output coordinate values, and one in which use is made of a position sensing device referred to as a PSD (an analog device in which an output voltage corresponding to the position of the spot is obtained).
By way of example, the specification of Japanese Patent Publication (KOKOKU) No. 7-76902 discloses an apparatus in which a beam spot produced by a parallel beam of visible light is sensed by a video camera to detect the coordinates of the beam spot and, at the same time, a control signal is sent and received in the form of infrared diffused light. Further, the specification of Japanese Patent Application Laid-Open (KOKAI) No. 6-274266 discloses an apparatus in which coordinates are detected using a linear CCD sensor and a special optical mask.
On the other hand, Japanese Patent No. 2503182 discloses the construction of an apparatus which uses a PSD and a method of correcting output coordinates in this apparatus.
Large-screen displays of improved brightness can now be used satisfactorily even in brightly lit environments and, as a result, demand for such displays is growing. A coordinate input apparatus combined with such a large-screen display also needs to be unaffected by extraneous light so as to be usable in brightly lit environments. In addition, devices which utilize infrared light are finding wider use as wireless communication means and there is a tendency for extraneous light to increase together with infrared light and visible light. Accordingly, that the apparatus be unaffected by extraneous light is an important characteristic.
However, with a coordinate input apparatus using a linear CCD sensor as set forth in the specifications of Japanese Patent Publication No. 7-76902 and Japanese Patent Application Laid-Open (Kokai) No. 6-274266, extraneous light can be suppressed only by an optical filter.
By contrast, with the coordinate input apparatus using the PSD as set forth in Japanese Patent No. 2503182, the intensity of light is frequency modulated and the modulated light is synchronously detected to thereby enable suppression of the effects of extraneous light. This means that making joint use of an optical filter provides a characteristic that is resistant to external disturbance.
The resolution of large-screen displays is being improved along with the brightness of such displays. Accordingly, though it is necessary to improve the resolution of the coordinate input apparatus as well, the PSD, which is strongly resistant to external disturbances, presents a problem in this respect. Specifically, the dynamic range of the sensor output voltage corresponds directly to the input range. Consequently, in a case where the overall coordinate input area is broken down into 1000 coordinates, for example, an S/N ratio of at least 60 dB is required. Furthermore, a digital correction of linearity error is essential, as set forth in Japanese Patent No. 2503182, and this necessitates a highly precise analog circuit, a multiple-bit AID converter and an arithmetic circuit. Further, since the S/N ratio of the sensor output signal is dependent upon the amount of light and the sharpness of the beam spot, satisfactory effects are not obtained solely through reliance upon the above-mentioned external-disturbance suppression property. This means that a bright, highly precise optical system is necessary. Such an optical system also is very high in cost.
The specification of Japanese Patent Publication No. 7-76902 discloses simultaneous use of a plurality of video cameras as a method of raising the resolution of a coordinate input apparatus that uses a linear CCD sensor. However, it is obvious that this approach leads to an apparatus of large size and high cost. Of course, using a single video camera having a large number of pixels would result in even a larger size and higher cost than in a case where a plurality of cameras are used. Further, in order to achieve a resolution greater than the number of pixels by image processing, an enormous quantity of image data must be processed at high speed. As a consequence, a very large and costly apparatus is required to achieve real-time operation.
The invention described in Japanese Patent Publication No. 6-274266 is contrived to obtain a high resolution by a special mask and signal processing. If there is little extraneous light and a good S/N ratio can be assured, an improvement in resolution is possible. In actuality, however, the image formed with a linear CCD sensor is linear and, in comparison with an area sensor in which a point image is formed, separation from extraneous light cannot be performed in a plane. The apparatus therefore is susceptible to the effects of extraneous light. Such an apparatus is applicable only in special environments in which there is little extraneous light.
Accordingly, an object of the present invention is to provide a coordinate input apparatus with which coordinates can be input at low cost and in highly precise fashion.
According to the present invention, the foregoing object is attained by providing a coordinate input apparatus for generating coordinates corresponding to a beam spot produced by a designating tool, the apparatus comprising: first image sensing means, which is constructed on a first coordinate axis by a first image-forming optical system and a first sensor array, for sensing the beam spot; and second image sensing means, which is constructed on a second coordinate axis by a second image-forming optical system and a second sensor array, for sensing the beam spot; wherein the first and second coordinate axes are orthogonal and image width of the beam spot formed by each of the first and second image-forming optical systems is larger than each pixel of the corresponding first and second sensor arrays.
The apparatus preferably further comprises adjustment means for adjusting the image width of the beam spot formed by each of the first and second image-forming optical systems.
The adjustment means preferably is a light diffusing plate.
The light diffusing plate preferably is placed on the side of the coordinate input screen facing the image-forming optical systems.
The light diffusing plate preferably has wavelength-selective transmissivity.
The light diffusing plate preferably is constituted by an infrared transmitting filter and a light diffusing film.
The light diffusing plate preferably is bonded to the first and second linear sensors.